A Humane Society bus ad in Washington D.C., similar to one proposed for Raleigh buses.

Credit Humane Society of the U.S.

The Humane Society of the United States is suing the Raleigh Transit Authority after the city rejected a bus advertisement showing pigs in cages.

The ad proposal meant for the outside of buses has photos of pigs in gestation crates with the words, "How would you like to spend the rest of your life in a space as small as a bus seat?"

The Raleigh transit board said the ad was too negative. But Kim Alboum of the Humane Society in North Carolina says the rejection violates the group's First Amendment right to free speech.

"The fact that they're saying that this ad is too negative? This is the reality. This is how these animals are living and what we're trying to do is a public information campaign about food practices," Alboum says.

"It's my hope and the hope of the Humane Society of the United States that it's disturbing enough that people think before they purchase their food; that they're purchasing their food where this cruel practice is not taking place. Gestation crates aren't necessary and they are a terrible, cruel practice in industrialized farming."

Raleigh transit officials would not comment on the lawsuit. They say bus advertising space is a limited public forum and the transit board reserves the right to reject ads it deems offensive. A non-profit that lobbies for food companies called the Center for Consumer Freedom says the ad does not represent actual conditions on pork farms.

The town of Chapel Hill recently revised its policy about bus ads after some residents were offended by dueling pro-Israel and pro-Palestine posters inside buses.